Question: Factor the following expression: $-3$ $x^2$ $-10$ $x$ $-3$
Solution: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-3)}{(-3)} &=& 9 \\ {a} + {b} &=& & & {-10} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $9$ and add them together. The factors that add up to ${-10}$ will be your ${a}$ and ${b}$ When ${a}$ is ${-1}$ and ${b}$ is ${-9}$ $ \begin{eqnarray} {ab} &=& ({-1})({-9}) &=& 9 \\ {a} + {b} &=& {-1} + {-9} &=& -10 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {-3}x^2 {-1}x {-9}x {-3} $ Group the terms so that there is a common factor in each group: $ ({-3}x^2 {-1}x) + ({-9}x {-3}) $ Factor out the common factors: $ x(-3x - 1) + 3(-3x - 1) $ Notice how $(-3x - 1)$ has become a common factor. Factor this out to find the answer. $(-3x - 1)(x + 3)$